As is known in the art, fasteners such as screws or pins are commonly used to secure or mate fractured bone sections. For example, a bone fracture can be held together during the healing process by means of a fastener such as a surgical screw. Such screws and the manner in which the screws are selected for use are known.
The process of inserting the screw involves first properly aligning the bone regions to be secured by the screw. A hole is then drilled through the bone regions. After the hole is drilled, the hole is tapped. The diameter of the screw to use, the size of the hole to drill, and the size of the tap are selected in accordance with a variety of well-known factors including but not limited to the size of the bone as is generally known by those of ordinary skill in the art.
One problem that arises, however, is that as a screw is being inserted into the hole in the bone, the screw hole may become stripped and the screw no longer obtains adequate purchase in the bone. A screw having inadequate purchase is sometimes referred to as a “spinner.” Reasons for this failure of fixation include poor bone quality, over-tightening of the screw and an error in the drilling or tapping of the screw hole (e.g. the tap itself cuts a new hole because the tap does not enter the drilled hole at the proper angle, the drill does not produce a straight hole, etc. . . . ). Other problems can also occur. For example, while the hole is being drilled, the fracture may shift which results in the existing hole being in a sub-optimal position.
One technique for addressing such a problems is to inject a bone cement (e.g. methylmethacrylate) into the screw hole and re-insert the screw while the bone cement hardens. This is a cumbersome and time-consuming process and also runs the risk of having the bone cement travel into the fracture site and impeding bony union. As a result of these and other risks, this technique is seldom utilized.
Another technique, which is more frequently utilized than the bone cement technique, is to attempt to drill another hole in a new position. However, this is not always possible due to a variety of factors including but not limited to the type and location of the bone fracture being repaired as well as the condition (e.g. quality) of the bone being repaired. A new drill hole may now create too large a drill hole or a drill hole with too thin a wall adjacent to the previous hole and again compromise or eliminate effective screw purchase. It is sometimes necessary to simply accept less than optimal fixation, or in some cases even substandard fixation, and possibly alter the post-operative rehabilitation plan. This may entail delaying weight-bearing or early motion and possibly compromising the patient's outcome.
In some cases, bone plates are used to obtain fixation and are commonly secured to the bone by screws. The use of bone plates can improve stability of the fractured bone gained by applying compression across the fracture site. Use of bone plates, however, can also make it more difficult to deal with spinners since the holes drilled in the bones must also match the locations of pre-drilled holes in the bone plates.
Another problem that arises is that osteoporotic bone or bone with abnormally thinned cortices due to failed fixation or previous infection may not allow lasting screw purchase in the bone. Thus, a surgical screw may first appear to have adequate purchase but due to bone or other conditions, the screw fails to maintain adequate screw purchase in the bone over a requisite period of time. Once the screw loosens in the bone, stability of the fractured bone gained by applying compression across the fracture site is quickly lost. A number of techniques for securing screws in a bone plate are described in U.S. Pat. Nos. 5,976,141 and 4,484,570.